In the context of marine seismic surveys, two types of seismic waves are of interest, namely P waves and S waves. P waves, or Primary waves, are compressional waves that are longitudinal in nature. These are pressure waves that can travel through any type of material including fluids. S waves, or Secondary waves, are shear waves that are transverse in nature and cannot travel any distance through fluids. They travel more slowly through solid materials than P waves, hence the name (“Secondary”). As S waves cannot travel through fluids, they can only truly be detected by receivers that are mechanically coupled to the seabed. Sophisticated processing techniques have been developed to make use of detected S and P waves to image subsea regions and in particular to detect and monitor hydrocarbon bearing formations.
Whilst, traditionally, P waves have been detected using arrays of sources and receivers towed in the water, both P and S waves can be monitored by measuring two physical effects at the seabed, namely pressure and particle velocity or particle acceleration. These measured physical effects are analysed using complex algorithms in order to detect and separate the P and S waves. Traditionally, seismic surveys have been conducted using arrays of so-called 4c sensors, each of which monitors four components, namely pressure and three orthogonal components of particle velocity (x, y and z), using a single hydrophone and three orthogonally-oriented geophones. More recently, it has been appreciated that additional data—including pressure derivatives in the horizontal plane (x and y directions) and the particle velocity derivatives in the horizontal plane (x and y directions)—can prove valuable in monitoring the P and S waves, resulting in higher quality (e.g. higher resolution) data. [The terms “gradients” and “derivatives are used interchangeably in the technical literature.]
It is noted that the horizontal particle velocity (in the water column) can be, and in practise normally is, derived from the horizontal pressure gradient measured at the seabed. Furthermore, the horizontal particle velocity's horizontal gradient can be derived from the derivative of the pressure gradients, that is the second order horizontal pressure gradient, and so forth.
To obtain additional data to improve the quality of the P-wave field, so-called 6c sensors are employed to measure six components, namely; pressure (p) and its first order spatial derivatives in the horizontal plane (dp/dx, dp/dy), and vertical particle velocity (Vz) and its spatial derivatives in the horizontal plane (dVz/dx,dVz/dy). In some cases, even more complex sensors may be used, e.g. 10c sensors to collect the 6c data plus four second order derivatives. These sensors do not necessarily need to be at the seabed, but could in principle be positioned anywhere in the water column. However, in order to measure S-waves, seabed coupled horizontal geophones or accelerometers are needed. These sensors are included as two of the components in traditional “4C seismic seabed recorders”. Here the four components (4C) are: Pressure, vertical particle velocity and the two orthogonal horizontal particle velocity sensors. When 6C and/or 10C sensors are combined or integrated with one or more seabed coupled 4C sensors, additional data is then available for improving the data quality of both S-wave and P-wave data.
FIG. 1 illustrates schematically two possible 6c sensor configurations. On the left is shown a configuration comprising 3×2c sensors, each comprising a hydrophone and a vertically oriented geophone. On the right is shown a configuration comprising 6×P sensors, each comprising a single hydrophone (NB. it is known that vertical particle velocity can be measured by making two separate vertically spaced pressure measurements).
A number of texts cover the principles of acquisition of marine seismic data (e.g., Sheriff and Geldart, 1995; Ikelle and Amundsen, 2005). There are several configurations of source and receiver distributions; those commonly used for petroleum exploration are (1) towed-streamer acquisition, where sources and receivers are distributed horizontally in the water column near the sea surface; (2) ocean-bottom seismic (OBS) acquisition, where the sources are towed in the water column and the receivers are on the seafloor; and (3) vertical-cable (VC) acquisition, where the sources are towed near the sea surface as in towed-streamer and OBS acquisition but the receivers are distributed in the water in a vertical array.
A particular case of the OBS acquisition involves the use of Ocean Bottom Nodes (OBNs), rather than the more conventional ocean bottom cables. OBNs are typically battery powered, cableless receivers placed in deep water. OBNs can be relatively easy to deploy and remove, and are especially suited for use in relatively congested waters where the deployment of streamers and ocean bottom cables is difficult. OBNs are typically deployed and recovered by Remote Operated Vessels (ROVs), using free fall systems and acoustic release to facilitate recovery, or using “nodes on rope” techniques where multiple nodes are attached to a rope with an acoustic release buoy at the end. These approaches are traditionally used to detect data that consists of both P and S waves. It should also be noted that there are significant advantages to collecting data (P waves) at or close to the seabed where recording conditions are quiet, being shielded from sea currents, and where conditions are good for low frequency data recorded by particle velocity sensors or accelerometers.
WO2011/121128 describes a method of providing seismic data (such as marine seismic data). A seismic source is actuated at a plurality of source locations. For each source location, a multicomponent seismic measurement is performed at at least one receiver location. A reconstructing method is applied to each multicomponent measurement to obtain additional data corresponding to source locations additional to the source locations at which the source was actuated. The additional data are output and/or used. WO2011/121128 proposes, by way of example, that this approach may be used in the context of OBN/OBS acquisition, i.e. where multicomponent (6c) receiver nodes are located on the seabed and the sources are towed in the water column by a surveying vessel.
Commercial Oil and gas discoveries are typically found in sedimentary structures defined as “traps”, where porous rocks are covered by tight cap rocks. The structures are visible on seismic images due to variations in elastic properties of the rocks. P and S wave derived images may have different expressions, because their response is determined by different elastic properties (shear stiffness and normal stiffness) and may produce images that can be both supplementary and/or complementary. For example, S waves may more easily “see through” overburden sediments containing gas, whereas P waves may be completely attenuated. Furthermore, S waves may be more responsive to fluid overpressure and associated Geohazards. On the other hand, P waves are more sensitive to fluid type (distinguish gas, oil, water) than are S waves. Using the combination of P and S wave responses, one can improve the overall geological and geophysical interpretation of the data, providing a more accurate estimate of location, size and volume (and pressure) prediction, and type of fluids presents in the reservoirs.
In order to produce high quality S and P images of the subsurface, advanced data processing of the recorded data is needed in order to filter out noise and “beam-form” or migrate the seismic energy to the right location (to the image point). Traditionally P and S data are imaged separately, and one assumes (requires) that the P-wave data set is free of S waves (also free of S to P converted data) and the S-wave data set is free of P waves. This may not be the case in practice, and therefore the results may be compromised.
Traditionally, the seismic industry relies upon processing/imaging steps to try to “wash out” and suppress any P wave/S wave crosstalk interference. Clearly, reducing the levels of noise in the input S and P wave data would improve the final image/or inversion results (for a given amount of effort/data size input and set of processing steps). Cleaner S and P input data, also would make processing/imaging/inversion using the wave equation more efficient, because a coupled solution (using full elastic formulation) may be split into separate processes, and run more efficiently with simpler formulations (for example scalar formulations).
A problem encountered with OBS systems is the interference that occurs between the two types of waves. For example, a detector mechanically coupled to the seabed and configured to detect S waves will pick up the effects of P waves propagating in the seabed. Although it may be possible to remove much of the effects of the early P waves by filtering based upon arrival time (P waves propagate faster through the subsea formation than do S waves) and apparent speed, not all of the effects can be removed, due to later P arrivals, for example as a result of reflections from different interfaces, ringing in the source signal, and overlapping P and S energy in time due for example to P-S conversion and reflections at or close to the seabed. Conversely, a detector located in the water just above the subsea surface and configured to detect the effects of P waves may be influenced by S waves. Although S waves do not propagate through the water, there will be some conversion of S waves and surface waves/interface waves (Scholte wave; S-wave travelling along the seabed) to P waves at the seabed. It is desirable to remove the effects of such converted S waves from the data collected by the P wave detector and remove the effects of P-waves on the S-detector.